A λ/4 plate has many uses in relation to an anti-reflection film and a liquid crystal display. The λ/4 plate has been widely and practically used. Though the name implies λ/4, most λ/4 plates give λ/4 at particular wavelengths.
Japanese Patent Provisional Publication Nos. 5(1993)-27118 and 5(1993)-27119 disclose a phase retarder in which a birefringent film giving high retardation and another birefringent film giving low retardation are laminated to arrange their optical axes perpendicularly crossed. If the retardation difference of those films is kept λ/4 in the whole visible wavelength region, the phase-retarder theoretically gives λ/4 in the whole visible wavelength region.
Japanese Patent Provisional Publication No. 10(1998)-68816 discloses a λ/4 plate giving λ/4 in a wide wavelength region. The disclosed λ/4 plate comprises laminated two films made of the same polymer, and one of the films gives λ/4 and the other gives λ/2 at the same wavelength.
Japanese Patent Provisional Publication No. 10(1998)-90521 also describes another wide-ranging λ/4 plate comprising laminated two polymer films.
Japanese Patent Provisional Publication No. 2000-137116 and International Patent WO00/26705 describe another phase-retarder and its application to a circularly polarizing plate and a liquid crystal display. The phase-retarder consists of a single polymer film, and the shorter wavelength it is measured at, the smaller retardation the phase-retarder gives.